What you have here is the typical largely useless (to brewers) fluff report put out by the water authority to reassure consumers that they aren't going to get typhus nor lead poisoning from the water. Arlington gets its water from the Corps of Engineers Dalecarlia plant. Go to http://www.dcwater.com/waterquality/test_results.cfm
. The first item there is a .pdf download of the District's water quality report. As they also take their water from the Dalecarlia Plant the results pertain to Arlington water as well.
Reports aside, lots of people do all grain in Arlington without much treatment or no treatment at all. It is the alkalinity which is of major concern and while it is a wee bit higher than one might like at 64 ppm as CaCO3 it is not much to worry about when starting out. This water is good for lots of styles of beer.
When you get more into it you might want to look at the Primer which will recommend cutting the water with RO water and using supplemental calcium chloride and possibly calcium sulfate. More and more people in the DC area are doing this. It makes better beers but the ones you make with the water straight out of the tap will certainly be decent.